Potential of Biochar Filters for Onsite Wastewater Treatment - MDPI

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Potential of Biochar Filters for Onsite Wastewater Treatment: Effects of Biochar Type, Physical Properties and Operating Conditions Luis Fernando Perez-Mercado , Cecilia Lalander, Christina Berger and Sahar S. Dalahmeh * Department of Energy and Technology, Swedish University of Agricultural Sciences (SLU), Box 7032, SE 750 07 Uppsala, Sweden; [email protected] (L.F.P.-M.); [email protected] (C.L.); [email protected] (C.B.) * Correspondence: [email protected]; Tel.: +46-18-67166; Fax: +46-18-673529 Received: 20 November 2018; Accepted: 7 December 2018; Published: 12 December 2018

Abstract: The potential of biochar as a filter medium for onsite wastewater treatment was investigated in five sub-studies. Sub-study 1 compared pollutant removal from wastewater using pine-spruce biochar, willow biochar and activated biochar (undefined biomass) filters. Sub-study 2 investigated the effects of particle size (0.7, 1.4 and 2.8 mm) on pollutant removal using pine-spruce biochar filters. In sub-studies 3 and 4, the effects of the hydraulic loading rate (HLR; 32–200 L m−2 ) and organic loading rates (OLR; 5–20 g biochemical oxygen demand (BOD5 ) m−2 ) on pollutant removal using pine-spruce biochar filters were investigated, while sub-study 5 compared pollutant removal in pine-spruce biochar filters and in sand. The removal of chemical oxygen demand (COD), total nitrogen (Tot-N), ammonium nitrogen (NH4 -N), phosphates (PO4 -P) and total phosphorus (Tot-P) was monitored in all sub-studies. All types of biochar and all particle sizes of pine-spruce biochar achieved a high degree of removal of organic material (COD > 90%). Removal of Tot-P and PO4 -P was higher in willow biochar and activated biochar (>70%) than in pine-spruce biochar during the first two months, but then decreased to similar levels as in pine-spruce biochar. Among the particle sizes tested, 0.7 mm pine-spruce biochar showed the lowest amount of Tot-P removal, while 2.8 mm pine-spruce biochar showed the lowest level of NH4 -N removal. Different OLRs and HLRs did not influence COD removal (94–95%). Pine-spruce biochar showed a better degree of removal of Tot-N than sand. In conclusion, biochar is a promising filter medium for onsite wastewater treatment as a replacement or complement to sand, achieving high and robust performance regardless of the parent material, particle size or loading conditions. Keywords: biochar filters; hydraulic loading rate; particle size; wastewater treatment

1. Introduction Diffuse pollution from onsite wastewater treatment systems (OWTS) is a major contributor to environmental pollution in both developed and developing countries. For example, around one million people in Sweden are not connected to a public sewerage system and are served by approximately 700,000 OWTS [1]. Up to half of these OWTS function inadequately, due to age and/or poor maintenance. Consequently, OWTS release up to 15% of the total phosphorus load to the Baltic Sea [2]. Moreover, around 70% of all waterborne disease outbreaks in Sweden have been traced to private drinking water wells near OWTS, with the most common cause of outbreaks being contamination with faecal bacteria from OWTS [3]. Similarly, countries in Latin America rely heavily on onsite wastewater treatment systems (e.g., Imhoff tanks, wetlands, stabilisation ponds) for the treatment of wastewater from small towns and villages [4]. However, a large number of

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these wastewater treatment plants often experience design, construction and operational difficulties, resulting in poor performance, which makes these plants a major source of environmental pollution [5]. A typical system for onsite wastewater treatment consists of a septic tank followed by a sand-filter trench or soil infiltration system [1]. Natural sand is a finite source and can be associated with clogging problems [6]. Moreover, production of crushed sand has a high energy demand for crushing and transporting the heavy product. Consequently, the main obstacles to using sand filters are a scarcity of well-graded sand in some regions and high transportation costs due to the high bulk density. Therefore, there is a great need for efficient, renewable, environmentally friendly and readily available materials to replace sand beds and increase the removal of wastewater pollutants. Biochar is a carbon produced by thermal decomposition (pyrolysis) of organic materials at an elevated temperature (300–800 ◦ C) in the absence of oxygen [7]. Besides biochar, pyrolysis of organic materials produces syngas, which is used as a renewable fuel for heat production [8]. Generally, biochar is characterised by having a large surface area (200–1000 m2 /g), low density and high porosity [9], which makes it an efficient adsorbent and good biofilm carrier. Due to the unique properties of biochar, there is growing interest in using it as a filter medium to enhance water and wastewater quality in onsite systems. Part of this interest is the recognition that the utilisation of biochar for wastewater purification can increase treatment efficiency and reduce the spread of contamination from hazardous chemicals in treated flow streams, compared with conventional soil and sand infiltration systems, and can also mitigate climate change effects through carbon sequestration [10]. The physical, chemical and structural properties of biochar (stability, pore size, pH, cation exchange capacity (CEC), ash fraction, specific surface area and mineral content) can vary greatly depending on the type of organic material used and the biochar production conditions (temperature, heating rate and oxidation) [7,11]. Previous studies have demonstrated the efficiency of biochar as an adsorbent and biofilm carrier for removing organic matter, surfactants, phosphorus (P) and nitrogen (N) from onsite wastewater and greywater treatment systems [12–15]. However, the effects of a biochar parent material on pollutant removal and the pollutant removal performance of biochar under different loading and operating conditions are not examined in these studies. The capacity of filters to remove pollutants differs between materials due to differences in characteristics such as porosity, specific surface area, reactivity, adsorption capacity and ability to promote biofilm development [16]. In addition, wastewater production in households varies on a daily, weekly and seasonal basis, which leads to variability in organic loading rates (OLR) and hydraulic loading rates (HLR) to the wastewater treatment systems. Under peak conditions, this can lead to a temporary breakdown of the infiltration system, a so-called episodic failure [17]. Thus, it is necessary for the infiltration bed to have the capacity to withstand variations in HLR and OLR and maintain resilient and steady treatment performance. Consequently, successful design requires knowledge about the capacity of the particular filter material to buffer high variations in water flow and organic loading. The overall aim of this study was to evaluate the potential of vertical flow biochar filters for onsite wastewater treatment, compared with sand filters, in regards to the removal of organic matter, phosphorus and nitrogen. Specific objectives were to: Assess the physical, chemical and hydraulic properties of different types of biochar and their importance for adsorption and biodegradation of wastewater pollutants. Secondly, to assess the performance of vertical flow biochar filters in removing organic matter, phosphorus and nitrogen under different organic and hydraulic loading conditions. Thirdly, to describe the effects of biochar particle size on organic matter and phosphorus removal, nitrification and denitrification. 2. Materials and Methods 2.1. Experimental Set-Up Pine-spruce biochar was obtained from Vindelkol AB (Umea, Sweden), willow biochar was obtained from a local farmer (anonymous), activated biochar was obtained from VWR (Stockholm,

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Sweden) and sand was obtained from Rimbojord (Sand & Grus AB Jehander; Stockholm, Sweden). Laboratory‐scale biochar and sand column filters were packed in acrylic columns of 5 cm (diam.) × Laboratory-scale biochar and sand column filters were packed in acrylic columns of 5 cm (diam.) × 55 55 cm (depth) (Figure 1) and operated under room temperature (20 ± 2 °C). cm (depth) (Figure 1) and operated under room temperature (20 ± 2 ◦ C). Influent Coarse biochar (2.5 cm)

Filter material (50 cm)

Coarse biochar (2.5 cm)

Effluent Figure 1. Schematic diagram of theof laboratory-scale filters (diam. cm) used for testing thetesting wastewater Figure 1. Schematic diagram the laboratory‐scale filters 5(diam. 5 cm) used for the wastewater treatment efficiency four media different filter media (triplicate columns): Activated biochar, treatment efficiency of four differentoffilter (triplicate columns): Activated biochar, non-activated non‐activated willow biochar, non‐activated pineand spruce biochar and sand. willow biochar, non-activated pine spruce biochar sand.

To address the different objectives of this study, five sub-studies were conducted: Sub-study 1 To address the different objectives of this study, five sub‐studies were conducted: Sub‐study 1 assessed the effects of physical, chemical and hydraulic properties of different types of biochar on assessed the effects of physical, chemical and hydraulic properties of different types of biochar on the the removal removalof ofpollutants pollutantsfrom from wastewater. Three types of biochar (non-activated biochar, wastewater. Three types of biochar (non‐activated willowwillow biochar, non‐ non-activated pine-spruce biochar and activated biochar of undefined origin) were used in filters activated pine‐spruce biochar and activated biochar of undefined origin) were used in filters operated −2 day−1 and an OLR of 15 g biochemical oxygen demand (BOD ) m−2 operated at aofHLR Lm −2 day −1 and 5 at a HLR 34 Lof m34 an OLR of 15 g biochemical oxygen demand (BOD5) m−2 day−1 (Table − 1 day1). (Table 1). Sub-study 2 investigated the effects of biochar particle size on pollutant removal using Sub‐study 2 investigated the effects of biochar particle size on pollutant removal using non‐ non-activated pine-spruce biochar. activated pine‐spruce biochar. Three particle sizes (0.7, 1.4 forpollutant pollutantremoval removal a HLR Three particle sizes (0.7, 1.4and and2.8 2.8mm) mm) were were investigated investigated for at at a HLR 34 L34 −2−2 day − 1 and an OLR 15 g BOD−2 m−2−1day−1 (Table 1). Sub-study 3 investigated the effects of −1 L mm day and an OLR 15 g BOD5 m5 day (Table 1). Sub‐study 3 investigated the effects of the OLR pollutant removal in biochar filters. Non‐activated pine‐spruce biochar (particle of d10size = 1.4of the on OLR on pollutant removal in biochar filters. Non-activated pine-spruce biochar size (particle −2achieve at two at OLRs, and 205 gand BOD day5−1m . To OLRs, these the filters were d10 mm) = 1.4 was mm)operated was operated two 5OLRs, 205 m g −2BOD day−1 . these To achieve OLRs, the −2 day−1, respectively. − 2 − 1 fed at HLRs of 200 and 34 L m Sub‐study 4 investigated the effects of increasing filters were fed at HLRs of 200 and 34 L m day , respectively. Sub-study 4 investigated the effects −1 of the HLR from 37 to from 200 L37 m−2today filter non‐activated biochar (d10 = 1.4biochar mm) of increasing the HLR 200−1 Lusing m−2 aday using a filter of pine‐spruce non-activated pine-spruce −2 −1 − 2 − 1 the keeping OLR constant (5 ±constant 2 g BOD(55 m ) (Table 1). Sub‐study compared the 5 (d10while = 1.4 keeping mm) while the OLR ± 2 day g BOD day ) (Table51). Sub-study 5 m 10 = 1.4 mm in both cases) in performance of non‐activated pine‐spruce biochar with that of sand (d compared the performance of non-activated pine-spruce biochar with that of sand (d10 = 1.4 mm in −1 −2 day−1. operated a HLR of at 37aLHLR m−2 day 5 g man −2OLR bothfilters cases) in filtersatoperated of 37and L man day−of1 and OLR of 5 g m−2 day−1 . Three replicate biochar filters were used for each biochar type and each particle size used in sub‐ Three replicate biochar filters were used for each biochar type and each particle size used in studies 1–4. Only two replicate sand filters were used in sub‐study 5. sub-studies 1–4. Only two replicate sand filters were used in sub-study 5. The filters in sub‐studies 1–4 were fed with synthetic wastewater, which was prepared by mixing The filters in sub-studies 1–4 were fed with synthetic wastewater, which was prepared by mixing real wastewater, nutrient broth and different types of detergents according to Dalahmeh et al. [18]. real wastewater, nutrient broth and different types of detergents according to Dalahmeh et al. [18]. The desired strength of the synthetic wastewater was achieved by adjusting the proportions of the Thedetergents desired strength of the synthetic wastewater was achieved by adjusting the proportions of the and nutrient broth to the amount of water. In sub‐study 5, real municipal wastewater detergents and nutrient broth to the amountsewage of water. In sub-study 5, real municipal wastewater collected from the Kungsängen municipal treatment plant (Uppsala, Sweden) was used to collected from the Kungsängen municipal sewage treatment plant (Uppsala, Sweden) was used feed feed the filters. The wastewater was prepared weekly and stored at 2–4 °C. All filters were fedtowith ◦ C. All filters were fed with their the their filters.intended The wastewater was prepared weekly and stored at 2–4 daily wastewater dose using single‐pass downflow over a period of 20–26 weeks intended wastewater dose single-pass downflow a period using of 20–26 weeks (Table (Tabledaily 1). Prior to feeding, theusing refrigerated wastewater was over homogenised a magnetic stirrer1). Prior to then feeding, the refrigerated wasthe homogenised a magnetic stirrer and then the and the required dose waswastewater pumped from refrigerated using container to a distribution container, required was at pumped from the refrigerated container to a distribution container, was whichdose was kept 20 ± 3 °C. Thereafter, when the feed temperature had acclimatised, it waswhich pumped ◦ keptfrom at 20the ±distribution 3 C. Thereafter, when thefilter feed using temperature hadpump acclimatised, it was pumped from the container to the a peristaltic or a computer‐based distribution system. Depending onthe thefilter sub‐study, filters were fed or with 25 mL of wastewater on threesystem. to 10 distribution container to using athe peristaltic pump a computer-based distribution occasionson per day, to give the HLR.fed Thewith dosing evenly distributed 24 h. Depending the sub-study, therelevant filters were 25 events mL of were wastewater on three to over 10 occasions per day, to give the relevant HLR. The dosing events were evenly distributed over 24 h.

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Table 1. Variables tested in sub-studies 1–5 to assess the performance of non-activated and activated biochar and sand filters for small-scale wastewater treatment. Specifications

Sub-Study 1

Sub-Study 2

Sub-Study 3

Sub-Study 4

Sub-Study 5

Type of material

(1) Non-activated willow biochar (2) Non-activated pine-spruce biochar (3) Activated biochar

(1) Non-activated pine-spruce biochar



(1) Non-activated pine-spruce biochar (2) Sand

Number of replicates for each medium and treatment

3

3

3

3

3

Effective size (d10 , mm)

1.4

0.7, 1.4, and 2.8

1.4

1.4

1.4

Hydraulic loading rate (L m−2 day−1 )

32

34

200 and 34

200 and 37

37

Organic loading rate (g BOD5 m−2 day−1 )

15-20

20

5 and 20

5

5

Chemical pollutants

COD, BOD5 NH4 , NO3 , Tot-N, PO4 -P, Tot-P

COD, BOD7 , NO3 , Tot-N, PO4 -P, Tot-P

COD, BOD7 , NH4 , NO3 , Tot-N

COD, BOD7 , NH4 , NO3 , Tot-N

COD, BOD7 , NO3 , Tot-N, PO4 -P, Tot-P

Filter operation period (weeks)

20

26

20

20

26

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2.2. Characterisation of Material Prior to packing the columns, the following properties of the biochar and sand media were determined: Loss on ignition, effective size and uniformity coefficient, specific surface area, surface composition and particle density of the material. After packing the columns, the following filter properties were determined: Bulk density, total porosity and hydraulic residence time. Constant-head hydraulic conductivity was determined only for the sand and activated biochar filters. Loss on ignition was determined by heating the dried material to 550 ◦ C for 4 h. Bulk density was determined by dividing the dry weight of the filter medium by the volume occupied by the medium (i.e., 5 cm × 55 cm). The particle density of biochar and sand, i.e., the ratio of total mass of solid particles to their total volume, excluding pore space between particles, was determined by dividing 25 g of sample by its corresponding volume. The volume of particles, excluding pores, was determined using the liquid immersion method, where the volume of deionised water displaced by the particles was measured. After adding water, air-filled pores were eliminated by gentle boiling of the mixture. The submerged particles were left to saturate for 24 h. The porosity of biochar and sand was determined as: ρ p = 100 × (1 − B ) ρP where p is porosity as a percentage of the total volume, ρB is the bulk density and ρP is the particle density. Constant-head hydraulic conductivity was determined according to Reference [19]. The specific surface area of biochar and sand was determined with the Brunauer-Emmett-Teller (BET) method, using a kaolinite sample with a defined area of 15,900 m2 kg−1 as standard [20]. The BET equation was used to calculate the specific surface area of biochar and sand based on measurements at 99,834 Pa and 20 ◦ C, where 1 mL of N2 gas corresponded to 2.86 m2 [20]. The residence time of water in the filter was determined after 10 days from start-up by adding a 100 mL pulse of NaCl tracer solution (10 g L−1 ) to the filters and then monitoring the electrical conductivity (EC) of the effluent as a function of time [21]. The mean residence time in the filter was defined as the time when 50% of the total tracer had eluted. Material composition, i.e., the internal structure, surface topography and surface chemistry of the non-activated pine-spruce biochar, activated biochar and sand was identified using elemental scanning electron microscopy (SEM). SEM micrographs and energy dispersive X-ray spectrographs (EDS) of the samples were obtained using a HITACHI TM-1000 scanning electron microscope, equipped with an Oxford Instruments EDX detector (Belfast, UK). To obtain reliable statistics in the elemental analysis, the value used for each point was the average of three individual measurements. The scanned surface was mapped by moving over the sample with steps of 10 µm. 2.3. Chemical Analysis Grab samples of influent and effluent were collected once a week throughout the experiments. The following chemical parameters were determined with a frequency of once or twice per week: Chemical oxygen demand (COD), biochemical oxygen demand (BOD7 ), total nitrogen (Tot-N), ammonium nitrogen (NH4 -N), nitrate nitrogen (NO3 -N), total phosphorus (Tot-P), phosphate (PO4 -P), electrical conductivity (EC) and pH. These parameters were determined using chemical kits and the prescribed methods (Table 2), which are in accordance with the standard APHA methods (APHA, 2007). The analytical quality was ensured by using control and standard solutions with known concentrations of the substance for every measurement series (Table 2).

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Table 2. Chemical kits and methods used for analysis of wastewater characteristics. Substance EC

Kit Name

Measurement Range

Electrical conductivity

pH

Control Solution Name and Value

Apparatus

mS cm−1

Calibration liquid: KCl 500 µS/cm

Conductivity Pocket Meter, Cond340i WTW, Germany

Standard Unit (SU)

Calibration liquid: pH 7 and pH 9

pH-meter Ino Lab pH Level 1, WTW pH-electrode Blueline 14 pH, Schott instruments

Potassium hydrogen phthalate solution 1.11769.0100, Merck 170 mg L−1 and Combi R1, Combicheck 20 1.14675.0001, Merck 750 ± 75 mg L−1

Thermoreactor TR 420, Merck, Germany Spectroquant NOVA 60, Merck, Germany Pipettor, VWR, Poland Analog Vortex Mixer, VWR, USA

Combi R1, Combicheck 20 1.14675.0001, Merck 12 ± 1 mg L−1

Spectroquant NOVA 60, Merck, Germany Pipettor*, VWR, Poland Analog Vortex Mixer, VWR, USA

Nitrate standard solution 1.19811.0500, Merck 1000 mg L−1


EN ISO 11905-1 (digestion)

Nitrate standard solution 1.19811.0500, Merck 1000 mg L−1

Thermoreactor TR 420, Merck, Germany Spectroquant NOVA 60, Merck, Germany Pipettor*, VWR, PolandAnalog Vortex Mixer, VWR, USA

Phosphate standard solution 1.19898.0500, Merck 1000 mg L−1

Thermoreactor TR 420, Merck, Germany Spectroquant NOVA 60, Merck, Germany Pipettor*, VWR, Poland Analog Vortex Mixer, VWR, USA

Phosphate standard solution 1.19898.0500, Merck 1000 mg L−1


Units

Standard Method

COD

Chemical oxygen demand

Spectroquant COD Cell Test (Hg-free) 1.09772.0001 and 1.09773.0001

10–150 and 100–1500

mg L−1

No standard, but Hg-free

NH4 -N

Ammonium

Spectroquant Ammonium Cell Test 1.14544.0001

0.5–16

mg L−1

EPA 350.1, US Standard Methods 4500-NH3 D, and ISO 7150/1

NO3 -N

Nitrate

Spectroquant Nitrate Cell Test 1.14764.0001

1–50

mg L−1

Total nitrogen

Spectroquant Nitrogen (total) Cell Test 1.147630001 and 1.00613.001

Tot-P

Total phosphorus

Spectroquant Phosphate Cell Test 1.14543.0001

0.05-5

mg L−1

EPA 365.2 + 3, APHA 4500-P E, and DIN EN ISO 6878

PO4 -P

Phosphate

Spectroquant Phosphate Cell Test 1.14543.0001

0.05-5

mg L−1

EPA 365.2+3, APHA 4500-P E, and DIN EN ISO 6878

Tot-N

10–150 and 0.5–15

mg

L−1

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2.4. Statistical Analysis An analysis of variance (ANOVA) at a 95% confidence level was used to assess differences in wastewater removal between: Different types of media, different particle sizes, different OLR values and different HLR values. When a statistically significant difference was found, a Tukey multiple comparison of means at 95% confidence level was carried out. All statistical analyses were performed using Statistica ver. 12 (Statsoft Inc., Tulsa, OK, USA). 3. Results and Discussion 3.1. Physical and Chemical Properties of Biochar and Sand The specific surface area of the pine-spruce biochar varied from 170 to 200 m2 g−1 but was considerably higher (1000 m2 g−1 ) in the activated biochar. It was significantly lower for the sand (0.152 m2 g−1 ). Specific surface area is an important parameter when evaluating the suitability of a material for use in wastewater filters, as a large area indicates high potential for the development of a widespread biofilm [22]. Within the biofilm, biological degradation, mineralisation of organic matter, nitrification and denitrification take place. Moreover, the larger the specific area, the higher the capacity for adsorption and the precipitation of various organic and inorganic pollutants. The SEM revealed longitudinal hollow tubes with high porosity, resembling the original wood structure in the non-activated pine-spruce biochar (Figure 2), as also reported previously [23]. The SEM of the activated biochar revealed random pore structure on the surface of the material and pores that appeared to be distributed all over the surface. The SEM images of the sand particles revealed a solid structure with limited occurrence of micropores and the fewest micropores of the three types of material studied. In line with these findings, all biochar (activated and non-activated) materials showed similar porosity (60–74%), which far exceeded that of the sand (35%). This means that a filter made of biochar would have better capacity to hold water in macropores than a sand filter and also better capacity to harbour biofilm in the pores without clogging or restricting aeration, which are both crucial aspects of wastewater treatment. The non-activated biochar can thus be expected to provide more suitable surface conditions for bacterial attachment and biofilm development than the other materials, which should lead to an efficient biological degradation of organic matter and nitrification. It is possible that the richness of micropores and nanopores on the surface of the activated biochar could induce clogging due to faster biofilm accumulation, including trapping of organic material during the infiltration process, compared with the non-activated biochar with its larger pores [24]. The biochar (activated and non-activated) filters also had substantially smaller particle density and bulk density than sand filters of the same particle size (d10 = 1.4 mm) (Table 3), making transportation and handling easier for biochar than for sand. Table 3. Properties of the activated biochar, willow biochar, pine-spruce biochar and sand filter materials used in column experiments. The hydraulic properties (porosity and mean residence time) were measured at a hydraulic residence time of 32 ± 7 L m−1 day−1 .

1

Non-Activated Willow Biochar

Non-Activated Pine-Spruce Biochar

Sand

1.5 and 2.8–5 0.6 >1000 560 1890 70.6

1–1.4 and 2.8–5 6.3

1.4–5

1.4–5

170–200 187

0.152 1690 2570 34

119 500

108

Filter Material

Activated Biochar

Particle size (mm) Air-dry water content (%) Specific surface area (m2 /g) Bulk density (kg m−3 ) Particle density (kg m−3 ) Total porosity (%) Water-filled porosity (%) Mean residence time (h) Hydraulic conductivity (cm h−1 )

270 740 63.3

72–74 48–53 87 1 ; 85 2 ; 66 3

0.5 360

Residence time of pine-spruce biochar filters with d10 = 0.7 mm. 2 Residence time of pine-spruce biochar filters with d10 = 1.4 mm. 3 Residence time of pine-spruce biochar filters with d10 = 2.8 mm.

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Residence time of pine‐spruce8 of 18 biochar filters with d10 = 1.4 mm. 3 Residence time of pine‐spruce biochar filters with d10 = 2.8 mm. 2

The EDS images of the non-activated pine-spruce biochar revealed a relatively low mineral The EDS images of the non‐activated pine‐spruce biochar revealed a relatively low mineral content (sodium (Na), magnesium (Mg), aluminium (Al), silicon (Si), chlorine (Cl), potassium (K), content (sodium (Na), magnesium (Mg), aluminium (Al), silicon (Si), chlorine (Cl), potassium (K), phosphorus (P), calcium (Ca), titanium (Ti) and iron (Fe) on the particle surfaces (Figure 2), with Ca phosphorus (P), calcium (Ca), titanium (Ti) and iron (Fe)) on the particle surfaces (Figure 2), with Ca being present in the highest proportions (13% w/w). Aluminium was present in proportions of 11–39% being present in the highest proportions (13% w/w). Aluminium was present in proportions of 11–39% in all samples, but this was probably due to contamination from the aluminium foil used to wrap the in all samples, but this was probably due to contamination from the aluminium foil used to wrap the biochar in the laboratory, and not from the biochar itself. No Fe or Mg was detected on the surface of biochar in the laboratory, and not from the biochar itself. No Fe or Mg was detected on the surface of the pine-spruce biochar, whereas the surface of the activated biochar contained substantial proportions the pine‐spruce biochar, whereas the surface of the activated biochar contained substantial of Fe (41%) and Ca (30%). The surface of the sand particles contained more Ca and Fe (15% and 21%, proportions of Fe (41%) and Ca (30%). The surface of the sand particles contained more Ca and Fe respectively) than the surface of the non-activated pine-spruce biochar, but less than the surface of (15% and 21%, respectively) than the surface of the non‐activated pine‐spruce biochar, but less than activated biochar. According to the supplier of the sand used in the studies, it had been mixed with 5% the surface of activated biochar. According to the supplier of the sand used in the studies, it had been lime. The presence of Ca, Fe, Al and Mg, which have an affinity for soluble reactive P, is an important mixed with 5% lime. The presence of Ca, Fe, Al and Mg, which have an affinity for soluble reactive feature of a filter medium, as adsorbed PO4 3− will precipitate to form surface complexes and hence be P, is an important feature of a filter medium, as adsorbed PO43− will precipitate to form surface immobilised [25]. Based on the elemental composition, activated biochar and sand can be predicted to complexes and hence be immobilised [25]. Based on the elemental composition, activated biochar and have the highest potential for P adsorption from wastewater. sand can be predicted to have the highest potential for P adsorption from wastewater.

Scanning electron electron microscope microscope image image of of the the surface surface of of(A) (A)non‐activated non-activatedpine‐spruce pine-sprucebiochar, biochar, Figure 2. Scanning (B) activated 1500. The The values values in in the the tables tables show activated biochar biochar and and (C) (C) sand. sand. Magnification Magnification factor factor × ×1500. percentage of the chemical composition standard deviation, deviation, nn == 3). 3). composition of of the the surface surface (mean (mean ± ± standard


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2

8 6 4 2

12

EC (mS/cm)

10 8 6 4 2

EC (mS/cm)

0

Percentage recovery

EC (mS/cm)

10

Percentage recovery

12

EC (mS/cm)

0

25%

0% 0 3 6 9 12 0 3 6 9 12 15 0% Tracer residence time (days) 0 3 6 9 12 15 Tracer residence time (days) 3 6 9 12 15 12 Tracer residence time (days) Tracer residence time (days) 100% 100% 10 (B) Willow non-activated biochar 80% (B) Willow non-activated biochar 8 80% 60% 6 60% 40% 4 40% 20% 2 20% 0 0% 0 3 6 9 12 150% 0 3 6 9 12 0 3 6 9 12 15 0 3 6 9 12 15 Tracer residence time (days) Tracer residence time (days) Tracer residence time (days) Tracer residence time (days) 12 100% 100% (C) Sand filters 10 (C) Sand filters 80% 80% 8 60% 60% 6 40% 4 40% 0

0

25%

2 0

0 0

072 72 144 216 288 144 216 288 360 Tracer residence time (hours) Tracer residence time (hours)

15

Percentage recovery

2

50%

Percentage recovery

4

4

20%

Percentage recovery

6

Percentage recovery

EC (mS/ cm)

9the of 18different filters had the longest mean hydraulic residence time among 9 of 18 biochar types with the same effective size (d10 = 1.4 mm) (activated biochar 4.9 days, willow biochar 4.5activated days, pine-spruce biochar 3.5 days), owing to the biochar activation process The biochar filters had the time among thecreating differentnano, micro The activated biochar filters had the longest longestmean meanhydraulic hydraulicresidence residence time among the different and macro-pores (Figures 3 and also appeared to be the case for pine-spruce biochar filters, biochar types with the same effective size4).(dThis mm) (activated biochar 4.9 days, willow biochar 10 = 1.4 biochar types with the same effective size (d10 = 1.4 mm) (activated biochar 4.9 days, willow biochar with effective size other 1.4owing mm, which had a mean hydraulic residence time of about 3.5 days 4.5 days, pine‐spruce biochar 3.5than days), to the biochar activation process creating nano, micro 4.5 days, pine-spruce biochar 3.5 days), owing to the biochar activation process creating nano, micro and macro‐pores 3 and 4). 2.9 Thisdays also for appeared to mm be the case for biochar filters, time in all for d10 = 0.7(Figures mm and about d10 = 2.8 (Figure 4).pine‐spruce The hydraulic residence and macro-pores (Figures 3 and 4). This also appeared to be the case for pine-spruce biochar filters, with effective other than 1.4 mm, which had athan mean timehad of about 3.5 days biochar size filters proved to be much longer inhydraulic the sand residence filter, which a residence time of only with effective size other than 1.4 mm, which had a mean hydraulic residence time of about 3.5 days for d100.5 = 0.7 mm and3C). about days retention for d10 = 2.8 mmin(Figure 4). The hydraulic residence all h (Figure The2.9 longer time the biochar can be attributed to itstime highinwater-holding forbiochar d10 = 0.7 mmproved and about 2.9 days for d10 = 2.8 mm (Figure The hydraulic residence time in all filters to be much longer in the sand filter,4). which had a residence timeof ofcontact only capacity and high porosity. A longthan hydraulic residence time extends the duration between biochar filters proved to be much longer than in the sand filter, which had a residence time of only 0.5 h 0.5 h (Figure 3C). The longer retention in theincreases biochar can attributed toofitsorganic high water‐holding wastewater and biofilm, whichtime in turn thebeprobability matter degradation or (Figure 3C). The longer retention time in the biochar can time be attributed to its high water-holding capacity capacity and high porosity. A long hydraulic residence extends the duration of contact between nitrification. and high porosity. A long hydraulic residence time extends the duration of contact between wastewater wastewater and biofilm, which in turn increases the probability of organic matter degradation or and biofilm,12which in turn increases the probability of organic matter degradation or nitrification. nitrification. 100% (A) Activated biochar 10 12 100% 75% (A) Activated biochar 8 10 75% 50% 6 8

EC (mS/ cm)

Water 2018, 10, 1835 The activated biochar Water 2018, 10, x FOR PEER REVIEW

0% 360 0

15

20% 0% 3 0 6 3 9 6 12 9 15 12 Tracertime residence Tracer residence (days) time (days)

15

Figure3.3. Response curves ofcurves thethe filter materials to addition a NaCl pulse of 10pulse gpulse L−1, measured Figure 3. Response offilter the filter materials toofaddition a NaCl tracer of 10 gg LL−1−, 1measured Figure Response curves of materials to addition of tracer aofNaCl tracer of , as electric conductivity (EC) in the effluent from filters of (A) activated biochar (×; n = 2), measured electricconductivity conductivity(EC) (EC)ininthe theeffluent effluentfrom fromfilters filtersof of(A) (A) activated activated biochar biochar(B) (×non‐ ;nn==2), 2), (B) nonasaselectric (×; ▲ activated willow willow biochar (; n = 3)(; ; nsand = 2).(Diagrams toDiagrams the left show EC inshow effluent (B) non-activated biochar ( and ; n (C) 3)sand and((C) (C) sand (N;;nn==2). 2).Diagrams the left show activated == 3) and ▲ toto the left ECEC in effluent time and diagrams to the right show percentage of tracer recovered time. All filter inagainst effluent against time and diagrams theright right showpercentage percentage traceragainst recovered against time.All filter against time and diagrams toto the show ofoftracer recovered against time. materials had an effective particle size (d10) size of 1.4–1.5 mm and amm hydraulic rateloading of 32 Lrate m−2 of All filter materials materials had an an effective effective particle size (d10 of1.4–1.5 1.4–1.5 mm and aaloading hydraulic loading rate of 32 L m−2 had particle (d and hydraulic 10))of −1 − 2 − 1 day . −1 32 L m day day . .

Sincemacropores macroporescontribute contributelittle little to to the the total surface area ofofa afilter [26], it can be argued Since total surface filtermedium [26], it can beitargued Since macropores contribute little to the totalarea surface area ofmedium a filter medium [26], can be argued that filter matrices with both higher surface area and porosity should have a higher proportion of of that filter matrices with both higher surface area and porosity should have a higher proportion

that filter matrices with both higher surface area and porosity should have a higher proportion of micropores than materials with less surface area and porosity. Thus, of the materials tested in the

Water 2018,10, 10,1835 x FOR PEER REVIEW Water 2018,

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micropores than materials with less surface area and porosity. Thus, of the materials tested in the

present haveaahigher higherproportion proportionofofmicropores micropores than non-activated presentstudy, study,activated activatedbiochar biochar should should have than non‐activated biochar and both bothshould shouldhave havea higher a higher proportion of micropores sand. biochar(willow (willowand andpine-spruce), pine‐spruce), and proportion of micropores thanthan sand. Thiswas wassupported supported by by the the observed observed differences time between thethe materials This differencesininhydraulic hydraulicretention retention time between materials (activatedbiochar biochar>> non-activated non‐activated biochar in in unsaturated flow takes place (activated biochar>>sand). sand).Water Watermovement movement unsaturated flow takes place mainlyinin smallest pores [27], reducing water velocity increasing the retention mainly thethe smallest pores [27], reducing thethe water velocity andand increasing the retention time.time. Higher Highersurface specific and surface and retention time increase the contact opportunities between contaminantsand specific retention time increase the contact opportunities between contaminants and biofilm/adsorption sites [27], leading to better pollutant removal performance than in materials other biofilm/adsorption sites [27], leading to better pollutant removal performance than in other materials with less specific surface and porosity. with less specific surface and porosity. 2.50 140%

A 2.00

B

Percentage recovery

120%

EC (mS/cm)

100%

1.50 1.00 0.50

80% 60% 40% 20%

0.00

0% 0.0

5.0

10.0

15.0

20.0

Tracer residence time (days)

25.0

0.0

10.0

20.0

30.0

Tracer residence time (days)

Figure curve and and(B) (B)recovery recoverycurve curve after addition a NaCl tracer Figure4.4.(A) (A)Response Response curve after thethe addition of aof NaCl tracer pulsepulse of 10 of g 10 g LL−1−,1measured , measuredasaselectric electricconductivity conductivity (EC) effluent from non-activated pine-spruce biochar (EC) in in thethe effluent from non‐activated pine‐spruce biochar filters of0.7 0.7(red (reddiamond), diamond), (black diamond) (blue filterswith withan aneffective effectiveparticle particle size (d10 1.41.4 (black diamond) andand 2.8 2.8 mmmm (blue 10))of − 2 − 1 −2 −1 diamond).All Allfilters filters had had aa hydraulic hydraulic loading . diamond). loadingrate rateofof32 32LLmm dayday .

3.2. 3.2.Treatment TreatmentPerformance Performance 3.2.1. 3.2.1.Influent InfluentCharacteristics Characteristics Pollutant in the the real real or or artificial artificialwastewater wastewaterused usedas as influent varied from Pollutant concentrations concentrations in influent varied from −1 ; 20 mg Tot-N L −1 and 4 mg Tot-P −1L−1 ) to high intermediate L−1 intermediate (approximately (approximately 330 330 mg mg COD COD L ; 20 mg Tot‐N L−1 and 4 mg Tot‐P L ) to high −1 78–95 mg Tot-N L−1−1 and 4 mg Tot-P −1L−1 ) (Table 4). The differences (approximately (approximately1230 1230mg mgCOD CODLL−1;; 78–95 mg Tot‐N L and 4 mg Tot‐P L ) (Table 4). The differences ininthe reflected differences differencesininorganic organicmatter matter concentration between greywater thequality qualityof ofthe the influent influent reflected concentration between greywater (wastewater from from kitchen, kitchen, shower shower and wastewater (wastewater and laundry laundry activities) activities)and andmixed mixedhousehold household wastewater (wastewaterfrom fromtoilet, toilet, kitchen, kitchen, shower could also reflect differences in in (wastewater shower and and laundry) laundry)[10]. [10].They They could also reflect differences wastewaterproduced produced water‐rich regions (e.g., Sweden) water‐scarce regions Jordan) wastewater inin water-rich regions (e.g., Sweden) andand water-scarce regions (e.g.,(e.g., Jordan) [28,29]. [28,29]. However, wastewater characteristics with factors suchusage, as water usage, household However, wastewater characteristics vary withvary factors such as water household activities and activities number of personconnected equivalents the system [17]. Rural with OWTS the numberand of the person equivalents to connected the systemto[17]. Rural areas with areas OWTS tend to yield tend to yield more concentrated wastewater than municipal areas with large‐scale wastewater more concentrated wastewater than municipal areas with large-scale wastewater treatment plants, treatment plants, particularly in water‐scarce regions [28]. particularly in water-scarce regions [28].

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Table 4. Concentrations of organic matter (BOD7 and COD) and nitrogen and phosphorus species in raw wastewater used to feed the filters in sub-studies 1–5. All concentrations are in mg L−1 . Specifications

Sub-Study 1

Sub-Study 2

Sub-Study 3

Sub-Study 4

Sub-Study 5

Type of material

(1) Non-activated willow biochar (2) Non-activated pine-spruce biochar (3) Activated biochar *




(1) Non-activated pine-spruce biochar (2) Sand

Effective size (d10 , mm) Hydraulic loading rate (L m−2 day−1 ) Organic loading rate (g BOD5 m−2 day−1 ) COD (mg L−1 ) BOD7 (mg L−1 ) NO3 -N NH4-N T-N (mg L−1 ) PO4 -P (mg L−1 ) Tot-P (mg L−1 )

1.4 32 15–20 1230–1140 490–630 * 1–3 3.7–11.0 78–95 2.6–3.2 3.6–3.8

0.7, 1.4, 2.8 34 20 1229 ± 320 629 ± 105 1.3 ± 2.5 11 ± 9 78 ± 27 3.2 ± 0.8 3.8 ± 0.7 * BOD5

1.4 200 5 325 ± 103 26 ± 10 17 ± 8 7±3 26 ± 8

1.4 34 20 1229 ± 320 629 ± 105 1.3 ± 2.5 11 ± 9 78 ± 27 3.2 ± 0.8 3.8 ± 0.7

37 5 496 ± 87 131 ± 50 6±6 30 ± 4 1.87 ± 0.94

200 5 325 ± 103 26 ± 10 17 ± 8 7±3 26 ± 8

1.4 37 5 496 ± 87 131 ± 50 6±6 30 ± 4 1.87 ± 0.94

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3.2.2. Organic Matter Removal 3.2.2. Organic Matter Removal Comparisons of non-activated pine-spruce biochar, willow biochar and activated biochar showed

Fraction of pollutants removed

that they all efficiently removed organic matter biochar, in termswillow of COD, with no Comparisons of non‐activated pine‐spruce biochar andsignificant activated differences biochar between (Figure 5). All biochars tested in had a similar surface porosity and showed the thatbiochar they alltypes efficiently removed organic matter terms of COD, witharea, no significant hydraulic residence which resulted performance in terms organic differences betweentime the (Table biochar3),types (Figure 5). in Allsimilarly biochars high tested had a similar surfaceofarea, porosity and hydraulic residence time (Table 3), which resulted in similarly high performance in matter removal. terms of organic matter Efficient removal ofremoval. organic matter (94–99% of COD) was also demonstrated for all particle sizes −1particle of organic (94–99% COD) was testedEfficient (0.7, 1.4 removal and 2.8 mm) whenmatter operated for 6ofmonths at aalso HLRdemonstrated of 34 L m−2 for dayall and ansizes OLR of −2 −1 −2 2.8 −1 (Figure 1.4 and mm) when operated for 6 months at a HLR of 34 L m day and an OLR of 20 20tested ± 5 g(0.7, BOD m day 6). However, there was a statistically significant difference between 5 ± 5 g BOD 5 m−2 day−1 (Figure 6). However, there was a statistically significant difference between the the 2.8 mm filter and the smaller sizes (0.7 and 1.4 mm) in terms of COD removal, with the coarser 2.8 mm filter and the smaller (0.7efficiency and 1.4 mm) inwith termsdof COD removal, with the coarser filter filter material displaying the sizes lowest (94% 10 = 2.8 mm, compared with 99% for d10 material displaying the lowest efficiency (94% with d 10 = 2.8 mm, compared with 99% for d10 = 1.4 mm = 1.4 mm and 0.7 mm). A larger particle size allows for larger macropores between the particles, and 0.7 mm). A larger particle size allows for larger macropores between the particles, which which increases the risk of wastewater breakthrough and rapid passage through the filter, giving less increases the risk of wastewater breakthrough and rapid passage through the filter, giving less contact time between the filter medium and organic matter in the wastewater and thus less efficient contact time between the filter medium and organic matter in the wastewater and thus less efficient treatment of wastewater. A finding supporting the occurrence of this process was that the hydraulic treatment of wastewater. A finding supporting the occurrence of this process was that the hydraulic residence time of the 2.8 mm filters was shorter (66 h) than that of the 0.7 and 1.4 mm filters (85 and residence time of the 2.8 mm filters was shorter (66 h) than that of the 0.7 and 1.4 mm filters (85 and 8787h,h,respectively). Another indication of insufficient contact time due to rapid water passage was respectively). Another indication of insufficient contact time due to rapid water passage was poor nitrification, resulting in higher NH4 -Nconcentrations concentrations effluent of the poorNH NH4 -N in in thethe effluent of the 2.8 2.8 mmmm 4‐N nitrification, resulting in higher NH4‐N biochar filters than the 0.7 and 1.4 mm filters (Figure 6). Although the difference in organic matter biochar filters than the 0.7 and 1.4 mm filters (Figure 6). Although the difference in organic matter removal removalwas wasstatistically statisticallysignificant, significant,the thelevel levelachieved achievedwith withthe the2.8 2.8mm mmfilters filters(94%) (94%)was wasstill stillhigh. high.The hydraulic retention time in the 2.8 mm filters was considered long enough to efficiently remove organic The hydraulic retention time in the 2.8 mm filters was considered long enough to efficiently remove matter. Organic matter degradation by biofilm activity is the dominant removal process in sand organic matter. Organic matter degradation by biofilm activity is the dominant removal process inand any other and the present study seemed to be limited by the area of sand and biofilter any other[30] biofilter [30]rate and in thethe rate in the present study seemed to be limited bysurface the surface the filter material. The organic mattermatter reduction achieved in the sand filters in in this study area of the filter material. The organic reduction achieved in the sand filters this study(90–97%) (90– 97%) was comparable the >90% reported previously for sand filters with 0.21 mm effectiveparticle particlesize, was comparable to theto >90% reported previously for sand filters with 0.21 mm effective as reported byand PellNyberg and Nyberg assize, reported by Pell [30]. [30].

1 0.8 0.6 0.4 0.2 0 COD

NH4-N

PO4-P

Type of wastewater pollutant Figure willowbiochar biochar(blue (bluecolumns), columns), non-activated pine-spruce Figure5.5.Performance Performance of non-activated non‐activated willow non‐activated pine‐spruce biochar(red (redcolumns) columns) and activated inin the removal of wastewater pollutants biochar activatedbiochar biochar(black (blackcolumns) columns) the removal of wastewater pollutants −2 −1 −1organic hydraulic loading Lm and loadingloading rate of 15 g BOD day−15. Effective 5m atataahydraulic loadingrate rateofof3232 L −2mday day and organic rate of 15 g−2BOD m−2 day−1 . size (d10size ) in (d all10materials was 1.4was mm. mean mean value value (n = 3) errorerror barsbars represent Effective ) in all materials 1.4Bars mm.indicate Bars indicate (n and = 3) and represent standarddeviation. deviation. standard

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100% 100%

Percentage removal

Percentage removal

120% 120%

80% 80% 60% 60% 40% 40% 20% 20% 0%0%

0.70.71.41.4 2.82.8 0.7 1.4 2.8 2.8 0.7 0.7 1.4 1.4 2.8 2.8 0.7 1.4 1.4 2.8 2.8 0.7 0.7 1.4 1.4 2.8 2.8 0.7 0.7 1.4 COD COD

Tot-N Tot-N

NH4-N NH4-N

Tot-P Tot-P

PO4-P PO4-P

Type ofof pollutant Type pollutantand andbiochar biocharparticle particle size size Figure 6. Performance of pine-spruce biochar filters with effective particle sizes (d10 ) of 0.7, 1.4 and Figure 6. Performance of pine‐sprucebiochar biocharfilters filterswith with effective effective particle particle sizes (d ofof 1.4−and Figure sizes (d10L10))m −0.7, 20.7, 1 and 2.8 mm6.inPerformance the removalofofpine‐spruce wastewater pollutants at a hydraulic loading rate of 34 day1.4 and −2−2day−1−1and an 2.8 mm in the removal of wastewaterpollutants pollutants at−a1ahydraulic hydraulic loading loading rate of 34 LLm 2.8 mm in the removal of wastewater at rate of 34 m day and an − 2 an organic loading rate of 20 ± 5 g BOD5 m−2 day . Bars indicate mean value (n = 3) and error bars −2 day organic loading rate of 20 ±g 5 gBOD BOD day−1−1. .Bars Bars indicate indicate mean mean value (n == 3) and error bars organic loading rate of 20 ± 5 value (n 3) and error bars 5 5mm represent standard deviation. represent standard deviation. represent standard deviation.

Percentage removal Percentage removal

Removal of COD in the non-activated pine-spruce biochar was similar for OLRs of 5 ± 2 Removal of COD pine‐sprucebiochar biochar was was similar for OLRs and 2020 −2in −non‐activated 1 (95 and 99%, Removal of COD in thethe non‐activated pine‐spruce OLRsofof55±±2 2of and and 20 ± 5 g BOD respectively), with ansimilar effluentfor concentration about 5 m−1 day ± 5 g BOD5−2m−2 − day (95 and 99%, respectively), with an effluent concentration of about 10 ± 3 mg −1 (95 and 99%, respectively), with an effluent concentration of about 10 ± 3 mg ±105 ± g BOD m day 3 mg−15COD L 1 in both cases (Figure 7). Despite the small difference in percentage removal of COD L in both cases (Figure 7). Despite the small difference in percentage removal of COD, the rate −1 −2 day 1 was significantly COD Ltheinrate both cases (Figure 7).OLR Despite the small difference in−percentage removal of COD, rate COD, of removal at an of 20 ± 5 g BOD m A the possible 5 −2 −1 of removal at an OLR of 20 ± 5 g BOD5 m day was significantly higher. A possiblehigher. explanation for −2 day−1 was significantly higher. A possible explanation for 5 of removal at an OLR of 20 ± 5 g BOD m explanation is that the higher organic load to the biofilm developing this is thatfor thethis higher organic load provided moreprovided substrate more to the substrate biofilm developing on the surface this is that the higher organic load provided more substrate to theto biofilm developing on the surface on of the surface of the biochar. When the flux of organic matter biofilm increases, the biological the biochar. When the flux of organic matter to biofilm increases, the biological activity of the of the biochar. When the flux of organic matter to biofilm increases, the biological activity of the activity of the microorganisms stimulated [31],also and thereby also the mineralisation rate of[32]. organic microorganisms is stimulatedis [31], and thereby the mineralisation rate of organic matter It microorganisms is stimulated [31], and thereby also the mineralisation rate of organic matter [32]. matter [32]. It should also bethe noted that filters the biochar had a high capacity to buffer should also be noted that biochar had a filters high capacity to buffer variations in variations loading It should alsoconditions, beasnoted the biochar had amaintain high to buffer in loading in loading as by their ability to high COD removal rates fluctuations with average conditions, shownthat byshown their ability tofilters maintain high CODcapacity removal rates withvariations average conditions, as shown by their ability to maintain high COD removal rates with average fluctuations fluctuations in theload. organic The performance of the biochar was also similar of 25% in of the25% organic The load. performance of the biochar filters wasfilters also similar under the under two −2 d− 1 ),significant of the organic load. The performance of the biochar also similar underofthe two −2 d HLRs tested (34 and Lm ), with no effects orfilters trends in removal COD. the25% twoin HLRs tested (34200 and 200 L−1 m with no significant effectswas orpercentage trends in percentage removal −2 −1 HLRs tested (34 and 200 L m d ), with no significant effects or trends in percentage removal of COD. of COD. 120% 120% OLR 5 100% OLR 20 5 OLR 100% 80% OLR 20 80% 60% 60% 40% 40% 20% 20%0% 0%

COD

COD

Tot-N Type of wastewater pollutant

PO4-P

Tot-N PO4-P Type filters of wastewater pollutantparticle size (d10) of 1.4 mm in Figure 7. Performance of pine‐spruce biochar with an effective removal of wastewater pollutants at organic loading rates of 5 ± 2 and 20 ± 5 g BOD5 m−2 day−1. Bars Figure 7. 7. Performance Performanceof of pine-sprucebiochar biocharfilters filters with with an an effective effective particle particle size size (d (d1010))of of1.4 1.4mm mmin in Figure indicate mean value (n =pine‐spruce 3) and error bars represent standard deviation. −2 day−1 . removal of wastewater pollutants at organic loading rates of 5 ± 2 and 20 ± 5 g BOD m 5 day−1. Bars removal of wastewater pollutants at organic loading rates of 5 ± 2 and 20 ± 5 g BOD5 m−2 Bars indicate = 3)error and bars errorrepresent bars represent standard deviation. indicate meanmean valuevalue (n = 3)(nand standard deviation.

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3.2.3. Nitrogen The different types of biochar filter did not show statistically significant differences in terms of nitrogen removal (as NH4 -N) under the same conditions: i.e., HLR 32 ± 7 L m−2 day−1 , OLR 20 ± 5 g BOD5 m−2 day−1 and particle size 1.4 mm (Figure 8A). The removal of Tot-N in pine-spruce biochar filters under a HLR of 32 ± 7 L m−2 day−1 and an OLR of 20 ± 5 g BOD5 m−2 day−1 was initially high (90%. In biochar filters, organic matter removal is achieved immediately when the filter is taken into operation, while sand filters are less efficient in the initial phase. Despite the biochar filters not being designed for enhanced denitrification, they achieved an intermediate to high (50–88%) level of removal of nitrogen, depending on the type of biochar and the loading rate. The biochar filters removed 12-fold more nitrogen than sand filters operated under the same conditions. A biochar particle size of 1.4 mm seemed to provide the highest removal rate under a hydraulic loading rate of 37 L m−2 day−1 . Non-activated willow biochar and activated biochar removed >86% of PO4 -P, while pine-spruce biochar removed only 62% and sand removed only 75–83%. Most biochar filters and the sand filter showed a declining level of the removal of phosphorus over time. In summary, willow and pine-spruce biochar could be considered suitable materials to replace or complement sand filters in onsite wastewater treatment systems, as they can efficiently remove organic matter and ammonium from wastewater. However, the long-term performance of these materials needs further investigation. Author Contributions: L.F.P.-M. and C.B. collected the data; S.S.D. carried out the data analysis and wrote the manuscript; C.L. and L.F.P.-M. revised it. Funding: This research was funded by the Swedish Research Council Formas, Swedish Agency for Marine and Water Management (Havs-och vattenmyndighet), Swedish International Development Cooperation Agency (SIDA) and Foundation Olle Engkvist Byggmästare. Acknowledgments: We gratefully acknowledge Sven Smårs for his technical help and Gulaim Seisenbaeva for help in the scanning electron microscopy imaging. We thank Björn VInnerås, Mikael Pell for their valuable feedback. We thank Morgan Macaud, Amandine Diot and Maxime Leong Hoi for their help in running the experiments. Conflicts of Interest: The authors declare no conflict of interest.

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